An interesting site ranking English clubs by seasons spent in the top flight.
https://www.myfootballfacts.com/england_footy/football-league/seasons-in-top-flight/
More successful in the past doesn't mean bigger.
The only sensible way to define the size of a football club is by its current ability to sign the best players.
The biggest clubs have a gravity that relates to their size. The best players are always drawn inexorably towards the biggest clubs.
In modern terms, and in the context of the OP regarding holding on to our players, club size is purely based upon money.
RB Leipzig were founded just 11 years ago, but are probably considered bigger than all but the big four in England.
They have already managed a Champions League semi final...